Optical parametric amplification (OPA) is a general process whereby a sufficient intensity of light at one wavelength (or frequency), the pump wavelength, is used to establish optical gain at other (longer) wavelengths in a material having a non vanishing nonlinear susceptibility. (References 1 to 4) OPA is conceptually similar to parametric amplification processes that were well-known to occur at radio frequencies. However, the physical process underlying OPA differs fundamentally from the radio frequency counterpart, due in part to the difficulty in obtaining highly coherent (long coherence length) light sources and materials and conditions under which light waves at different wavelengths can strongly interact.
The relatively weak nonlinearities of optical materials made OPA impractical until the advent of very intense optical sources brought about by the invention of the laser. Like excited laser media OPA's exhibit optical gain and amplify light.
In standard laser, (gain) media energy is supplied by an external source, e.g., a flashlamp, an electron current, or another laser. (References 3 and 5) In OPA, the energy to produce gain is supplied by an optical pump source, to date always a laser, but here the pump laser light is coherently coupled to the output light fields produced by the OPA.
The coupling and the gain established in the OPA gain medium (nonlinear material) depends on the intensity and phase of the pump field. In a suitable nonlinear material, usually a birefringent crystal with a nonvanishing second order nonlinear susceptibility, optical gain (and thus amplification) is established at two wavelengths when the pump beam is characterized by a single wavelength. These two wavelengths, conventionally referred to as the "signal" and the "idler" wavelength, are such that their corresponding frequencies when added are equal to the pump frequency (conservation of energy). The "signal" and "idler" waves are symmetric in that one photon of each is created from each photon of pump light that is converted by the OPA process. While their wavelengths usually differ, they need not (the degenerate case). Practically there is no physical distinction between what is termed signal and what is termed idler except that "signal" usually refers to the wavelength most desired, if there is one.
Useful gain, gain established over path lengths longer than the order of the wavelength, occurs only for those propagation directions within the crystal where the pump beam and the signal and idler beams can propagate and stay in phase. Optical materials generally exhibit a property called dispersion in which the refractive index varies with wavelength. Normally, shorter wavelength light waves propagate in these materials at slower velocities than longer wavelength light waves. Consequently, as they propagate light beams with different wavelengths move in and out of phase and there is no appreciable build up of gain even for exceedingly high intensities of the pump beam.
However, in certain birefringent materials, for particular temperatures and propagation directions, the difference in refractive index between ordinary and extraordinary waves can be used to offset the effects of dispersion at certain wavelengths, and phase coherence can be maintained as the beams propagate. This is called phase-matching. Under phase-matched conditions appreciable gain can build up at the signal and idler wavelengths if the pump beam is sufficiently intense. If optical feedback is provided, for example by using mirrors reflective at the idler or signal wavelengths (singly resonant condition) or at both wavelengths (doubly resonant condition) optical parametric oscillation or lasing can occur. The first demonstration of an optical parametric oscillator (OPO) was by Giordmaine and Miller in 1965 (Reference 1, cited hereinabove).
Although avidly pursued during the 1970's and 1980's, optical parametric amplification and oscillation did not quickly mature into broadly useful devices. This was due primarily to the lack of suitable nonlinear materials that could support efficient parametric frequency (energy) conversion processes at optical intensities below those that caused (optical) damage. Nonetheless, some research grade devices were demonstrated in a variety of nonlinear crystals. These devices produced tunable light from fixed frequency (wavelength) pump sources.
Tuning is possible because the pump beam can induce optical gain at any pair of wavelengths that conserve (photon) energy, and gain can build up if phase matching can be maintained. Since the index of refraction of a light beam polarized along the extraordinary direction in a birefringent material depends on its direction of travel with respect to the principal optical axes, the spatial orientation of the OPA material with respect to the propagation and polarization directions of the pump beam determines the wavelengths (and polarizations) of the signal and idler that can experience gain build up for any fixed pump wavelength. Tuning of the signal and idler wavelengths for fixed pump wavelength is most typically achieved by changing the angular orientation of the OPA crystal with respect to the direction of propagation of the pump beam. An alternative method of achieving phase-matching and of tuning an OPA or OPO is to vary the temperature of the crystal. Other methods that use external means, such as electric or magnetic fields, pressure, etc., to affect the crystal's refractive indices also could be exploited to phase-match and tune.
Although to date many useful devices have been based upon this method, a number of problems inherent to the angle tuning method arise. Since the crystal angle must be changed during tuning, the beams (pump, signal, and idler) can undergo angular deflection. This leads to requirements for precise angular compensation, since the stability of (singly and even more strongly doubly resonant) OPO's are critically dependent on the alignment of these beams. Angle tuning also requires larger aperture nonlinear crystals to be used than would otherwise be needed. Moreover, tuning the crystal requires precise fine mechanical motion. This is hard to do reliably and reproducibly and cannot be done quickly.
When the pump beam is tuned the OPO changes wavelength essentially immediately (within the build up time of the output fields, typically nanoseconds). In the past, reported and/or demonstrated OPA and OPO devices were based on fixed frequency pump beams. (References 5 to 9) Tunable laser sources were converted to other (tunable) wavelengths using nonlinear mixing processes such as sum- and difference frequency generation. The distinction between OPA and nonlinear mixing processes is discussed by reference 2, Byer and Herbst, Non-Infrared Generation, Topics In Applied Physics (Series), vol. 16, pp. 81-137 (1977).
Until the 1980's, the only tunable lasers sufficiently developed to be commercially useful were lasers based upon optically pumped solutions of dyes. While a few tunable solid state lasers, based on color centers in crystalline hosts, were in use these were largely limited to low power and/or low temperature operation. In the 1980's a new class of tunable solid-state lasers emerged. These lasers are based largely on (transition) metal ion impurities that interact vibronically with their crystal host. (Reference 10, J. C. Walling, Tunable Paramagnetic-Ion Solid-State Lasers", in Tunable Lasers Topics In Applied Physic (Series) vol. 59, pp. 331-398 (1987)). The first of these tunable vibronic solid-state laser materials that was sufficiently robust for commercial application was alexandrite. Subsequently many other tunable vibronic solid-state materials were discovered, and a few of these have been developed for use in commercial laser devices. These materials include titanium sapphire, and chromium doped lithium calcium aluminum fluoride [LiCaAlF6 or LlCAF] and, lithium strontium aluminum fluoride [LiSrAlF6 or LiSAF]., [Ref. 10, 11]. These tunable solid-state lasers are capable of producing high intensity tunable light efficiently.
Also emerging in the middle and late 1980's were new classes of nonlinear materials, particularly those of the orthophosphate and borate families (esp. potassium titanylphosphate KTP and Barium borate BBO). (Reference 7, J. T. Lin and C. Chen, "Choosing a Non-Linear Crystal", Lasers and Optronics, vol. 6, No. 11, pp. 59-63 (1987)). These new materials had sufficiently high nonlinear susceptibilities and damage thresholds to permit OPO's of more advanced designs to be developed. [Ref. 7, 12]. Further advances in nonlinear materials, tunable laser materials, and improved methods for pumping [Ref. 13-15] and tuning [Ref. 16] tunable laser materials using semiconductor diode lasers will provide improved embodiments of the inventions described below.
It is an object of the present invention to provide broadly tunable laser output with maximal coverage over the ultraviolet, infrared and visible spectral regions.
A further object of the present invention is to provide for simplified designs of tunable OPA's and OPO's.
A still further object of the present invention is to provide a narrow intense laser having narrow line width that is broadly tunable or suitable at a particular frequency.